1. Technical Field
This invention relates to the art of making bipartite connecting rods by cracking, and more particularly to forgeable wrought steel materials which enhance the characteristics of the connecting rod as well as facilitate cracking.
2. Discussion of the Prior Art
Connecting rods have been made bipartite for some time by sawing the enlarged end of a connecting rod into two pieces and reassembling the sawed pieces with bolts (see U.S. Pat. Nos. 1,831,325 and 2,371,400). Fracturing is a more attractive alternative because it allows the irregular jagged ridges on the cracked plane to facilitate more exact rematching of the pieces. One of the earliest patented approaches to cracking forged wrought steel rods is disclosed in U.S. Pat. No. 2,553,935 (Parks et al, 1951). The steel used for such cracked connecting rods was strong, tough, and ductile, usually containing 0.4-0.5 carbon with alloying elements added to improve hardenability if the part is to be quenched and tempered and sulfur added to improve machinability in many cases. Examples of such alloys are SAE 1151, 1141, and 1541; 1151 is a resulfurized grade that contains essential elements of (by weight percent): 0.48-0.55, carbon, 0.70-1.00 manganese, 0.04 max. phosphorous, 0.08-0.13 sulfur, and the remainder being essentially iron except for about 1% other residuals; 1141 is also a resulfurized alloy containing similar amounts of sulfur, phosphorus, and residual elements, but with a lower carbon content, 0.37-0.45%, and a higher manganese content, 1.35-1.65; 1541 has essentially the same composition as 1141 except that it is not resulfurized, having a content of 0.05 max. and has a tempered martensite microstructure. SAE 1151 is used for connecting rods which are air-cooled to a ferrite-pearlite microstructure after forging. 1141 and 1541 have better hardenability, conferred by the higher manganese content, and are used when rods are to be quenched and tempered. Regardless of the above alloy chosen, connecting rods are typically made to a hardness range of 76-88 HRG (16-26 HRC) to facilitate subsequent machining after heat treatment. In this hardness range, typical tensile values for any of these alloys are; tensile strength 100-125 ksi, yield strength 75-95 ksi, elongation 18-28%, and a charpy impact value of 60-100 ft/lbs.
Since wrought steel forging connecting rods heretofore have not been inherently brittle, some technique must be used to weaken the material at least in the cracking plane. To encourage cracking of such ductile wrought steel in the Parks et al patent, the cross-sectional area of the cracking plane was reduced by saw kerfs and drilled holes. This avoided using heat treatment to make such ductile steels more brittle which would discourage ease of subsequent finish machining and would degrade the toughness of the connecting rod. Other brittlyzing techniques were avoided, such as the use of cryogenics which proved to be exorbitantly expensive. An extension of the Parks idea of reducing the cross-sectional area is also shown in a later U.S. Pat. No. 4,693,139, employing dual V-notches.
Cracking of such ductile rods has been attempted by a variety of techniques, such as (i) use of a wedge-actuated, expandable mandrel, fitting within the large bore of the connecting rod, to apply tension across a predetermined plane perpendicular to the longitudinal axis of the rod, (ii) use of conical force fingers jammed into holes along the cracking plane, (iii) use of single-step, continuous pull-apart fixtures, or (iv) use of impact against the side of the rod.
What is needed is a crackable low ductility steel forging alloy that retains all of the other advantages of good machinability, strength, and toughness, characterized by the SAE 1151, 1141, and 1541 steels. Applicants are unaware of any steel alloy that has been specifically formulated to enhance crackability of forged wrought steel connecting rods.